Technology: Taking the grind out of superconductors

As superconductors move ever closer to the marketplace, the speed, precision
and cost of manufacturing techniques is becoming vital. Now a chemical process
that speeds up a key stage in the production of high temperature superconductors
promises to help provide the large quantities of the materials that industrial
applications will require. The new method, which has been developed at the
University of Queensland, Australia, produces superconducting ceramics of
much higher purity than in the past and opens the way to their widespread
application, say researchers at the centre for microscopy and microanalysis.

The superconducting materials are ceramics composed of complex oxides
of copper and other metals. They could be immensely valuable to power industries
because they lose their resistance to electricity when cooled to the relatively
high temperature of liquid nitrogen (-196 °C), which is cheap and readily
available. Before the discovery of these materials was announced in late
1986, superconductivity was only known to happen within a few degrees of
absolute zero (-273 °C), a temperature that can be achieved only when
the much more costly liquid helium is used.

Potential industrial applications for these superconductors – which
range from the storage and transmission of energy to magnetically levitated
trains – will require tonnes of the material. Until now they have been difficult
to make, particularly in large quantities, because they require precise
ratios of raw materials containing metals such as bismuth, strontium and
copper oxides to be ground together, before being heated to high temperatures
to produce batches of about 10 grams of superconductor.

The process developed at the Queensland centre is intended to provide
industry with enough ceramic to make long lengths of superconducting wire
continuously. ‘The powders had to be of high quality and able to be made
quickly in large amounts,’ Mackinnon says. ‘We couldn’t wait for months
of grinding. We had to go straight to a method with an industrial application.’

Advertisement

The new process replaces the grinding stage with a chemical process
that takes place in aqueous solution. Ian Mackinnon, the centre’s director,
says that soluble salts of the metals are mixed together with oxalate in
precise amounts. The oxalate salts of the metals that make up the specified
superconductor then precipitate out as a mixture, present to within four
or five per cent of the required ratios.

The new process is twice as quick as grinding and the centre is already
making batches of up to a kilogram of powder to fill orders worldwide, Mackinnon
says. Within three years he hopes to have a working pilot plant capable
of making batches at least ten times larger. The superconductor is heated
first so it can be shipped as powdered oxides: on heating they form the
superconducting copper oxides. The powders sell for between A $1500 ( £700)
and A $2000 per kilogram – about 25 to 30 per cent less than in the US,
according to Mackinnon. There is also a time saving: the new process is
twice as quick as grinding.

Customers for the superconducting material include the Showa Mining
Company of Japan, Concurrent Technologies in the US, BICC in Britain and
its Australian subsidiary MM Cables. Most are still at the prototype stage
of research and development for applications of the superconductor.

Since the process occurs in solution, the precipitate is malleable and
can be moulded or cast in defined shapes, rather like working with clay.
The group has devised a technique for doing this. Customers are then sent
the dry ceramic in the required shape. This should allow the production
of superconducting parts for electric motors, or of specially shaped magnetic
shielding devices. The process also allows the ceramics to be laid down
as a thick film on other material.

Mackinnon says it was clear that the superconducting properties of
the ceramics depended on their structure. So the secret to cracking the
problem of large scale production was to create a multidisciplinary team
which included expertise in analysing microstructure and linking it to the
chemical process which produced it.